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We use braces {} for variable expression like
NAME="test"
FILE_NAME=${NAME}file
But I don't understand in which scenarios we use brackets ()
Say nslookup $(hostname) works only with () brackets.
Can someone explain?
948
{} has absolutely no meaning to bash , so is passed unmodified as an argument to the command executed, here find . On the other hand, ; has a specific meaning to bash . It is normally used to separate sequential commands when they are on the same command line.
Within the -exec stuff an argument of {} means "insert the file name here". So if the files were "foo" and "bar" it would execute "ls -a foo" then "ls -a bar".
Different programming languages have various ways to delineate the start and end points of a programming structure, such as a loop, method or conditional statement. For example, Java and C++ are often referred to as curly brace languages because curly braces are used to define the start and end of a code block.
The end of the variable name is usually signified by a space or newline. But what if we don't want a space or newline after printing the variable value? The curly braces tell the shell interpreter where the end of the variable name is.
Minor nitpick first:
[]
()
{}
""
''
`` (Same as the tilde ~ key)Braces are used in BASh scripts for complex variable expansion. Consider string concatenation:
STR="hello"
STR2=$STR
STR2 evaluates to "hello". What if you wanted to make it something like "helloWorld". Doing something like STR2="$STR2World" won't work, so you use braces, ie: STR2="${STR}World".
As for brackets, they are used, similar to the backtick, `, which expands the text between them as the text output from a command.
What if you wanted to store the current time as a string?
STR2=$(date)
Now STR2 stores the string "Thu May 7 09:32:06 PDT 2015".
Additionally, you can use parentheses to execute something in a subshell, which will potentially affect your environment, PID, etc. Very useful for cases where you want a "throwaway" environment with having to track/restore environment variables, directories via pushd/popd instead of cd, etc.
146
Using parentheses ( executes something. There happens to be a program named hostname - so $(hostname) will execute it.
try which hostname to see where that program resides
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